The present invention is directed to treatment of contaminated liquids and, more particularly, to ozonation of organic and inorganic constituents in aqueous solutions.
Water from a variety of sources, such as industrial wastewater, groundwater, and the like, often contains unacceptable levels of a variety of organic and/or inorganic contaminants. The treatment of many of these contaminants is cumbersome and extremely expensive. One example of such a contaminant is methyltetra-butylether (MTBE). It is anticipated that over the next twenty years the United States will spend billions of dollars cleaning up MTBE pollution from leaking underground storage tanks (UST) at retail gasoline facilities. MTBE is considered to be a major health hazard and has contaminated numerous public drinking supply wells. The current environmental system of choice to treat MTBE is air stripping.
Air stripping involves a phase transfer of the target constituents from a liquid media to a gas media. Because the target constituents are removed rather than destroyed, air stripping systems are susceptible to off-gas discharge and typically require treatment of vapor phase constituents. Moreover, air stripping techniques are expensive, due in part to large power requirements needed for a blower for the stripping tower. Air stripping is particularly expensive for treating MTBE because an air stripping system needs to be designed that is almost three times the size of that required to treat traditional organic contaminants, such as benzene.
One chemical oxidation technique that has been used for treating contaminated water employs ultraviolet light and hydrogen peroxide to form hydroxyl radicals. A critical limitation of this technique is the ability of the ultraviolet to penetrate the liquid stream to react with the hydrogen peroxide to form the hydroxyl radical. This technique also requires significant energy to operate effectively, and UV lamp strength degradation over time lowers the efficiency of the system. This degradation can be caused by lamp life and also fouling of the lamp glass caused by hardness, suspended solids or other surface coating mechanisms that may be associated with a particular water or wastewater stream.
Another chemical oxidation method involves using a single oxidant at atmospheric pressure. For example, hydrogen peroxide is a strong oxidizer that can be used for treating contaminated groundwater. In a reaction known as the Fenton reaction, hydrogen peroxide can be mixed with a metallic salt such as ferrous sulfate to produce a free radical, which breaks bonds in the hydrocarbon molecule in an exothermic reaction to produce a low-free-energy state, generally comprising production of carbon dioxide and water.
In situ systems employing Fenton-type reactions are described, for example, in U.S. Pat. No. 4,591,443 to Brown and in U.S. Pat. No. 5,611,642 to Wilson. Each includes mixing the Fenton reactants prior to introduction into the soil and groundwater. U.S. Pat. Nos. 5,286,141 and 5,520,483, both to Vigneri, describe a remediation method and system that includes a pre-acidification of the ground water prior to a sequential introduction of the Fenton reactants, wherein hydrogen peroxide is added after an injection of ferrous sulfate at a high concentration.
Some water treatment processes have employed ozone in a pressurized system. For example, U.S. Pat. No. 6,197,206 to Wasinger describes a process for purifying MTBE contaminated water by treatment with air and/or ozone. The process utilizes a plurality of pressurized tanks with sufficient pressures to cause the ozone to maintain micro-sized bubbles. The pressure in the contact tanks is between about 3 psi and 30 psi, and preferably is about 20 psi. A gas/air stripper strips residual ozone from the water, and the residual gas is delivered to a destruction unit for removing any remaining ozone, MTBE, and oxidation products. The pressure is said to create micro-sized bubbles, which allow for more surface contact with the MTBE contaminated water and provide a faster rate of reaction.
In another approach, Mausgrover et al. U.S. Pat. No. 5,427,693 discloses a modular apparatus and associated method for generating ozone and for transferring the ozone into contaminated water. A venturi is connected to ozone generating means, and an infusion chamber is connected downstream from the venturi. A pump circulates water from a process tank through the venturi to the infusion chamber. The combination of the venturi and infusion chamber is said to produce a high mass transfer ratio of ozone into the water. The pressure maintained in the infusion chamber and the dimensions of the chamber are said to produce a headspace above the inlet that contributes to the formation of a vast number of relatively small bubbles.
There remains a need for a more cost-effective process for treating contaminated liquids. It would be desirable to develop a process that effectively converts contaminants by ozonation, while minimizing the quantity of ozone and other expensive reagents used.
The present invention is directed to a process and apparatus for treating contaminated liquids by reaction with ozone in a pressurized vessel. Ozone is injected into a contaminated liquid to form a gas/liquid. The mixture is injected into a mixing chamber. The mixing chamber provides turbulent gas/liquid contact. The mixture is flowed from the mixing chamber into the pressurized reactor vessel, in which a pressure of at least about 45 psig is maintained.
In an alternative embodiment of the present invention, a contaminated liquid, such as contaminated groundwater, is treated in situ by ozonation. Influent liquid is fed into a pressurized vessel. Ozone is injected into a recirculated liquid stream to form a gas/liquid mixture. The gas/liquid mixture is mixed with recirculated headspace gas from the vessel to form a reactive mixture having an air-to-liquid ratio from about 0.25:1 to about 10:1. The reactive mixture is injected into an isolated mixing chamber. The reactive mixture is flowed from the mixing chamber into the pressurized vessel, in which a pressure of at least about 45 psig is maintained. The reactive mixture is injected into the ground using a pressurized well injection system.
The process of the present invention can yield significant operating cost savings over presently available technologies, e.g., by providing significant reductions in chemical and/or activated carbon capital costs and significant reductions in disposal and handling costs. Preferred embodiments of the present invention also provide relative ease of operation, by avoiding the need for cumbersome and expensive air stripping systems and the associated need for active carbon disposal or regeneration. Long-term operating costs can be reduced by virtue of the lower power requirements of an ozone generator, for example, compared to that of a blower required for an air stripping tower. The process of the present invention is effective independent of the clarity of the contaminated liquid, unlike conventional UV/hydrogen peroxide systems, because light is not required for the oxidizer. When hydrogen peroxide is used as a second oxidizer, ozone and hydrogen peroxide are both dissolved in the liquid to form hydroxyl radicals.